System of variable amplification



NOV. 5, 1935. M|TCHELL ET AL 2,019,577

SYSTEM OF VARIABLE AMPLIFIGATION Filed Oct. 2, 1934 GA/N DECREASEI? VAR/O-REPEA TER GA //v INCREA SER lkll /& F 4mm i 36 GAIN T CHANGE DISABLE/Q By L I A TTORNEV Patented Nov. 5, 1935 UNITED STT mum:-

SYSTEM OF VARIABLE AMPLIFICATION of New York Application October 2, 1934, Serial No. 746,592

4 Claims.

The present invention relates to a four-wire transmission system with automatic variation of the gain in the transmitting branch of the system.

An object of the invention is an improved control of the variable gain mechanism to give improved overall operation of the system for talking or similar signaling.

In four-wire systems using a volume-operated gain adjusting device in the transmitting branch, difiiculties arise in making the device respond to control by the transmitted speech so that it may make the necessary adjustments to insure proper volume under all conditions, while at the same time preventing operation of the device by echoes or other currents that might adjust the device to a gain entirely unsuited to the speech that is in process of transmission.

It has been customary to disable the controls of the gain adjusting device when energy is present in the receiving side of the circuit in order to prevent the device from changing its gain during receiving times. This prevents the device from changing its gain under control of echoes.

A difficulty not obviated by this control is that produced when the talker on the transmitting side of the circuit, in which the gain adjusting device is located, makes short monosyllabic replies from time to time as he is listening to the opposite talker. Experience shows that these are of much lower volume than his sustained talk and they consequently tend to set the transmitting gain to a value much higher than is required for his sustained speech. As a rule, these short monosyllabic replies occur within brief pauses in the speech of the opposite talker.

In this invention there is a control which is relatively less responsive to sudden impulses of static than to speech and which, upon responding interrupts the gain increase control of the gain adjusting device for a considerable period. This prevents these monosyllabic replies from unnecessarily increasing the gain of the vario-repeater.- By making this control less responsive to static than to speech, the gain adjusting circuits for the vario-repeater are able to take control quickly at all times except immediately following speech received from the opposite talker.

The nature of the invention and its various objects and features will be more fully understood from the following detailed description taken in connection with the accompanying drawing.

The single figure of the drawing shows, in simplified schematic form, one terminal of a fourwire circuit comprising transmitting line 10 and receiving line H. A two wire line I2 is shown connected to the lines Hi and llthrough the usual hybrid coil H and balancing network N. Transmitting line H] contains a vario-repeater 13, the purpose of which is to maintain constant 5 volume on the line it] on the output side of the repeater for speech of widely varying volume incoming from line H2. The difference in volume on line [2 may be caused by difierence in length of line l2 leading eventually to the subscribers l0 station or by difference in loudness of different talkers or of the same talker at different times or to other causes. This vario-repeater l3 may be of any suitable type but will be assumed to be of the type disclosed and claimed in United 15 States patent to Hogg and Doba, No. 1,853,974 granted April 12, 1932 to which reference may be made for a more detailed disclosure.

The gain setting of vario-repeater I3 is controlled by the speech that is being transmitted, 20 provision being made for increasing the gain if the speech is below its prescribed minimum volume and for decreasing the gain if the speech exceeds permissible volume, as described more fully in the patent referred to. The gain is a 25 function of the charge on condenser I 5 which, in series with battery it is connected between the grids and cathodes of repeater it. A gain in creaser circuit is provided comprising filter I! for passing the important frequency components 30 of speech but discriminating against noise, an amplifier-detector l8 and relay l9. Under certain circuit conditions to be described later, when relay i9 operates, relay 20 operates to connect the positive pole of grounded battery 2! across condenser l5 to increase its positive charge and raise the gain of amplifier l3.

A gain decreaser is shown comprising vacuum tube 25 having its grid circuit connected to line I B on the output side of vario-repeater l3 through 40 input coil 25 and its anode-cathode circuit connected across condenser IS. The bias battery 21 makes the grid so far negative that, in the absence of impressed speech from 26, the cathodeanode impedance of tube 25 is substantially infinite and the tube does not shunt off the charge on condenser l5. Waves of sufficiently high amplitude in the grid circuit lower the cathodeanode impedance and cause a reduction in charge on condenser i5.

If the gain of vario-repeater I3 is high enough and accordingly does not need to be increased, the gain increase disabler circuit comprising gasfilled tube 30 and relay 32 operates under control of waves impressed on the grid through input coil 3|. When relay 32 operates, it opens the energizing circuit for relay 29 to prevent that relay from operating under control of relay l9. At its front contact this relay shunts out a resistance in the input circuit to increase the sensitivity of the gain increase disabler circuit.

By means of the gain increaser, the gain decreaser and the gain increase disabler, the gain of the vario-repeater i3 is maintained under control of the transmitted speech to keep the volume on its output side substantially constant. Usually, the vario-repeater comprises more than a single stage and a second amplifier 33 is shown, the gain of which may be controlled concurrently with that of repeater l3 over a branch of the grid lead.

In a volume operated gain control system of the type described, it is important to prevent false settings of gain under control of waves other than the speech that is being transmitted out over line It. An amplifier-detector 36 and relay 3'! are provided to prevent the vario-repeater l3 from adjusting on echoes. Whenever there is speech energy in line H, relay 3? is operated under control of amplifier-detector 36 to disable both the gain increaser and gain decreaser so that echoes from line l2 produced by the speech received over line H are unable to change the gain adjustment. This general provision against false setting on echoes is disclosed in application for United States Letters Patent filed April 30, 1932 by S. Doba, Serial No. 608,362. It is accomplished in the present disclosure by relay 38 which operates when relay 37 operates, opens the energizing circuit for relay 29 and, at its other armature, places a short circuit across the input of the gain decreaser 25.

It has been found that with so much of the system as has been thus far specifically described, a difficulty exists in that the vario-repeater is subject to a false gain adjustment by weak monosyllable replies by the talker on line l2. In an ordinary conversation, a train of words spoken by one party is frequently punctuated by short sounds from the other party indicating merely that such party is still on the line and is comprehending what is being said or, for example, is in a general way assenting. These short responses such as yeah or uh-huh are uttered in a much weaker voice than is used for sustained talking. Consequently, if the vario-repeater is allowed to adjust its gain to the volume of those weak sounds, it will be liable to be overloaded by the sustained talk when that occurs, until a new adjustment of the gain can be made.

The present invention solves that difiiculty. Observations show that these short replies are interspersed in the speech of the opposite talker at slight pauses that admit of time to utter them. They occur mostly within less than two and a half seconds after a fragment of speech by the opposite talker. In accordance with the invention, a circuit is provided which disables the gain increaser circuit for a period of two and a half seconds after the cessation of a steady talking period by the opposite talker.

This is accomplished in the circuit of the drawing by providing relays 4B and 4|, the operation of which will now be given. When relay 31 responds to receiving line energy, relay 4|) energizes in series with relay 38. Relay H is normally energized through the back contact of relay 4! and upon energization of the latter, current for a time continues to flow through winding of relay 3! and into condenser 42, charging the latter.

After a time interval determined by the capacity of condenser 42 and the resistance of the circuit, relay 4| releases placing a short circuit across the input to the gain decreaser circuit 25 and opening at its lower contact the negative polar- 5 izing circuit for the grid of the gas-filled tube 30 to permit that tube to break down and energize relay 32. Previous to the release of relay 4|, a circuit existed from negative battery El, front contact of relay 4|, resistance 45, positive battery 4'3 and ground, by which the bias on tube 30 was held negative by battery 2?. When relay 4| was energized, as described, the battery 2? was cut off and battery 43 then made the grid of tube 30 positive. Resistance M is very large and may, for example, be of the order of a megohm.

Relay 32, in operating, opens the circuit for relay 2!! making the gain increaser circuit ineffectual.

After relay 31 releases at the end of a talking period, relays 40 and GI both restore to the condition shown in the drawing but relay 32 remains held up for a two and one-half second interval on account of the slow-release circuit comprising condenser 45 and resistance 46. If talking is resinned in circuit during this interval, relays 31 and 46 energize and relay. 4| releases and. prevents the release of relay 32 with the qualification that relay 4| requires about one-tenth of a second to release, for a reason to be given presently. This relay operates in about one-fiftieth of a second.

Relay 38 is made slow to release by the condenser and resistance combination shown associated with its winding, the release time being about one-twentieth of a second. It will be observed that the gain can be quickly decreased after received speech ceases but cannot be increased until after the release time of relay 32.

Gain settings in response to weak echoes are 40 prevented by relay 38 which, as stated, is quick to energize and has a hangover time of onetwentieth of a second, sufficient to allow for echo decay. The circuit from the back armature of relay 38 for disabling the gain decreaser can be used where there is a tendency for echoes to be stronger than the speech transmitted over circuit In. In practice, it has been found sufficient, in some cases at least, to omit this connection and rely on relay 4| to disable the gain decreaser. 5 0 This can be done by severing the connection at the points X--X.

If it were not for static energy operating the anti-singing (vodas) relays frequently on a circuit such as thetransatlantic radio system, for 5;; example, relay 4| could be made quick to release. With frequent static impulses, if relay 4| were allowed to release quickly, the slow-release relay 32 might be held operated so much of the time on account of its long hangover time, that required gain increases could not be effected. Such gain increases might be needed, for example, to allow the talker on line l2 to get proper control of the voice-operated relays in the anti-singing circuit which may be assumed to follow the portion of 5 the circuit shown in the drawing. By making relay 4| slow to release, it is made to discriminate against static in favor of sustained speech waves.

In a circuit not exposed to static, it will be seen from what has ben said that a single control com- 7 prising relay 40 and relay 4| without its slowrelease feature could be made to suiiice, in which case relay 38 could be omitted.

What is claimed is:

1. In a four-wire telephone system comprising a transmitting line and a receiving line, a variorepeater in the transmitting line, means controlled by speech in the transmitting line for varying the gain of said repeater, and means to reduce improper gain adjustments by said gain varying means comprising means controlled by sustained speech in the receiving line for preventing energy in the transmitting line from changing the gain of said repeater over pauses long enough to exclude the effect of relatively weak substantially monosyllabic responses.

2. In a four-wire telephone system comprising a transmitting line, a receiving line, a variorepeater in the transmitting line for maintaining substantially constant output volume, means controlled by the speech energy being transmitted for varying the gain of said repeater to maintain such substantially constant volume, and a plurality of means responsive to energy in the receiving line for preventing changes in the gain of said vario-repeater, one of said last mentioned means comprising a quick response, slow-release circuit for preventing echoes from affecting the gain of said repeater and the other having a much longer release time for preventing incidental weak responses occurring in the transmitting line from aiiecting the gain.

3. The system of claim 2, in which the said other of said last-mentioned means is relatively slow to operate whereby it is substantially unresponsive to static but responsive to speech.

4. In a four-wire telephone system having a transmitting line and a receiving line, a variorepeater in the transmitting line having a gain increaser controlled from its input and a gain decrease and gain increase disabler controlled from its output, whereby substantially constant output volume of said vario-repeater is maintained under control of speech that is being transmitted, said gain increase disabler when operated having a hangover time of the order of sec0nds,'quick-operate means controlled by enery in the receiving line for disabling the gain increaser for preventing gain increase under control of echoes, and slow-operate means responsive to sustained energy in the receiving line for operating the gain increase disabler, whereby quick short responses in the transmitting line are prevented from causing increase of gain of said vario-repeater.

DOREN MITCHELL. LIONEL SCHOTT. 

